The present invention generally relates to drop-out and/or error compensation apparatus, and more particularly, to apparatus for providing such compensation in a digital data stream of a PAL format video information signal.
The present invention is intended for use with a video recording and reproducing apparatus such as a magnetic tape recording and reproducing apparatus, although other types of recording and reproducing apparatus may advantageously incorporate the present invention as will be apparent from the following description. The present invention effectively corrects picture impairments that are caused by unsuccessful recovery of data upon reproducing, from digital video recorders and particularly digital video magnetic recorders. At the present time, FM video recorders which use digital time base correcting apparatus during the playback of the recorded signal are limited to using direct replacement information from previous horizontal video lines to correct what are commonly referred to as drop-outs, wherein an imperfection in the magnetic tape or other problem results in the loss of the signal during reproducing for very short intervals. When the replacement information from the previous lines are used to substitute values for current video information that is defective or missing, the use of the replacement information from previous horizontal video lines leads to timing displacements due to the fact that the correction signal is not derived from the close proximity spatial information around the point of interest that is to be corrected.
More recent research and development has been directed toward digital recorders which does not employ FM recording, but in fact records digital words that represent samples that were taken of the analog video information signal. The digital words are recorded and reproduced and subsequently converted to analog signals. There are significant advantages in recording digital signals as opposed to FM recording. Among the advantages are the fact that during playback or recording, the video signal is not degraded by head-to-tape spacing losses, Moire, noise and velocity errors. As will be evident from the description of the present invention, the digital signals permit correction to be accurately generated by arithmatic means from the preceding line, i.e., the line above the line of interest, and the succeeding line, i.e., the video line below the point to be corrected. This close proximity spatial information around the point or digital word that is to be corrected significantly improves the positional timing of the correction signal.
Another inherent advantage of digital video recording and reproducing apparatus is that the sampling of the analog signal can be done at a precise phase relationship that is desired and is done before recording. Because the phase relationship is set during sampling and before recording, it will not be changed during playback by the presence of any velocity errors. The present invention is particularly adapted for use with a PAL format television signal which has a chrominance subcarrier at a frequency of approximately 4.43 MHz as well as the other characteristics of the PAL signal, including the presence of the U and V components which are used to advantage by the present invention. More particularly, the present invention utilizes a sampling rate of the PAL video signal at four times the subcarrier frequency of 4.43 MHz, which is a 17.7 MHz sampling rate. Also of significant importance is the fact that the samples are alternately taken along the U and V component vector axes so that for each subcarrier cycle, there are two U samples and two V samples. The sampling is done so that only the U component is sampled for two of the samples and only the V component is sampled for the other two samples for each subcarrier cycle and the absence of any V component for the U samples as well as the converse is used to advantage in generating a substituted value for the samples, as may be necessary if the data recovered during playback is either defective or missing.
By utilizing the sample values in closest proximity to the point that is to be corrected, the probability of obtaining a substitute value that is closest to the value of the point which is to be corrected or compensated for is increased. Whereas other compensation schemes have substituted a value that may have occurred at the same location from the immediately preceding subcarrier cycle, for example, or from the corresponding location from a preceding line or even from the same location of the same line from a preceding field, all of these schemes can produce a relatively incorrect value for the defective point that is to be compensated for, depending upon the particular content of the video image at the time as well as the movement that is occurring from field to field. Thus, if a point that is to be corrected was in a particularly dark portion of the image and if movement from dark to light has occurred for that point from the prior field to the present field, then a dark value will be substituted for the defective point even though the defective point should have been light by virtue of the movement having occurred in the image. Similarly, if a preceding cycle or preceding line is particularly close to a significant change in contrast or color, then the substitution of that value for the point to be corrected may result in an incorrect value being substituted for the defective point. It should also be understood that if the values that are to be substituted for current defective values are provided from a previous field, then it is necessary for a memory to be utilized that can store the data values for a complete field, which necessarily requires a memory of substantial capacity, particularly if the digital samples are comprised of eight bit words.
Accordingly, it is an object of the present invention to provide an improved apparatus for compensating for errors during recovery of data words of a video information signal, which apparatus has a high probability of generating an accurate replacement value for a defective word.
Another object of the present invention is to provide an apparatus for performing error correction which provides an accurate value for substitution of a defective value, and which does not require a memory of substantial capacity, it only requiring a capacity for storing data words for three horizontal video lines.
Another object of the present invention is to provide an apparatus for compensating for missing or defective data of the foregoing type, which utilizes an averaging technique utilizing similar types of samples from immediately adjacent samples in immediately adjacent horizontal video lines to thereby provide an average value that has a high probability of being accurate, notwithstanding significant movement in the video image or the presence of a distinct color or intensity contrast very close to the point which is being corrected.